Question: Jennifer used $\dfrac{2}{7}$ of a meter of ribbon to make bows for her cousins. Now, she has $\dfrac{10}{21}$ of a meter of ribbon left. How much ribbon did Jennifer start with?
Solution: To find the amount of ribbon Jennifer started with, we can add. $\frac{2}{7}$ $\frac{10}{21}$ amount of ribbon to start ribbon used ribbon left ${\dfrac{2}{7}} + \dfrac{10}{21}} = {\text{ amount of ribbon to start}}$ Our denominators need to be the same so we can add. What is the least common multiple for the denominators $7$ and $21}$ ? The least common multiple of $7$ and $21}$ is ${21}$. $\dfrac{{2}\times 3}{{7}\times 3} = {\dfrac{6}{21}}$ Now, we can add our fractions. ${\dfrac{6}{21}} + \dfrac{10}{21}} = \dfrac{{6} + 10}}{21} ={\dfrac{16}{21}}$ Jennifer started with $\dfrac{16}{21}$ of a meter of ribbon.